Using 7v To Power Unit Requiring +7v -7v

Discussion in 'The Projects Forum' started by CHays, Jan 14, 2011.

  1. CHays

    Thread Starter New Member

    Jan 14, 2011
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    Hi! I'm really glad to find this forum and I'm hoping someone can help me.

    I'm experimenting with my guitar electronics and I have a pedal that the specs say supplies 7v (in the 8 pin cable pin 1 is ground, and pin 2 is 7v). I would like to use that to supply power to a special pickup unit that the specs say requires +7v and -7v (in the 13 pin cable, pin 12 is +7v, pin 13 is -7v, and I think the cable shield acts as ground). How can I do this?

    I know very very little about electronics so if my question seems stupid it probably is. :) I'd appreciate any help or advice that will make this little project work.

    Just in case anyone is familiar with guitar stuff, here are some equipment specifics that may help. I'm using a Line 6 POD X3 Live and VDI cable (8 pin) to power a Line 6 Variax and I have a Roland GK-3 installed on the guitar that I need to power using the power supplied from the X3 to the Variax.

    I'll do my best to provide additional details or answer questions as needed.

    Thanks so much!
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    It might be do-able. How much current does your load (the pickup) require, and how much current is available from the +7v source?
     
  3. CHays

    Thread Starter New Member

    Jan 14, 2011
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    I'm afraid I don't know the answer to either of these questions. Sorry. I can measure the current at the source but I don't know how to tell what the pickup requires.

    I think it's possible somehow. There is a site that sells a product, www.rackvax.com, that does exactly what I'm trying to do. Link Here. I have not asked them, but I suspect they wouldn't give out their secrets. :) I'm using the same components to do the same thing, I just don't know how to wire it all up. I guess I could ask them...
     
    Last edited: Jan 14, 2011
  4. SgtWookie

    Expert

    Jul 17, 2007
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    If someone's come out with a product to perform the function, it's certainly possible to do.

    However, they probably invested a good bit of time to come up with a solution.

    So, you're paying for the time they worked on it to get the solution, which is reasonable.

    One possibility is the ICL7660S, which is a charge-pump IC that can create a negative polarity from a single positive polarity using just a few external capacitors. The output current is limited. I have no clue what current your pickup requires.

    Switching supplies are the most efficient, but are generally rather noisy. However, I don't know of a way to create a negative supply from a single positive supply other than using a switching supply.

    If the frequency of the switching supply is high enough, the noise should not be audible.

    You should put your location information in your profile. Click on the User CP link, scroll down about 2/3 of the way, and enter your country and state or province. That will help a lot when considering parts and vendor suggestions/recommendations.
     
  5. CHays

    Thread Starter New Member

    Jan 14, 2011
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    First of all, thanks for your help. I appreciate it and I've updated my location as you suggested. I've just placed an order with Digi-key for the ICL7660S so I can give that a try. My pedal is putting out 7.95v. I found a little more information that may or may not be helpful. I found a page describing a break-out for use with the Roland GK pickup. There is an image and a paragraph perhaps half way down the page describing the use of two nine volt batteries to power the pickup. Here is part of the description: "...It will not work with GK2A / GK3A type pickups or Roland ready guitars as you need to power the GK unit. This is typically +7 volts and -7 volts, but the spec sheet for the op amp gives a max rating of +/- 19 volts so a simple circuit with 2x PP3s will do fine for this..." The entire page can be found here: http://www.unfretted.com/loader.php?LINK=/profs/breakout
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    OK, good deal. :)

    You'll need at least 3 10uF capacitors. It would also be a good idea to put a 0.1uF cap from pin 8 (Vin) to ground.

    Since the ICL7660 is a switched-capacitor DC-DC converter, it's very efficient - however, you may find that there is some electrical noise due to it's oscillations. You may need to add a resistor between your 7v supply and Vin to isolate the noise.

    See the attached for the basic schematic for inverting your supply. I've used a Linear Technology part which is equivalent to the industry standard ICL7660.

    Note that the diode is to prevent "latchup" if the output accidentally goes above ground.
     
    Last edited: Jan 15, 2011
  7. SgtWookie

    Expert

    Jul 17, 2007
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    The attached schematic is very similar to the last one, however I've added a few capacitors and a couple of low-value resistors.

    The added resistors and capacitors will cause a very significant reduction in any noise generated by the 7660; probably on the order of -20dB
     
  8. CHays

    Thread Starter New Member

    Jan 14, 2011
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    I can't thank you enough! I'll get to work on it as soon as my part arrives.
     
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